In the manufacture of thin film resistor (TFR) type printheads for thermal ink jet pens, it has been a common practice to build up thin film printhead devices from a common insulating or semiconductive substrate such as glass or silicon. These devices typically include a surface insulating layer such as silicon dioxide, SiO.sub.2, formed on the silicon or glass substrate surface. A layer of resistive material such as tantalum aluminum, TaAl, is then deposited on the surface of the silicon dioxide insulating layer, and then a conductive trace pattern is formed on the surface of the resistive layer using conventional state-of-the-art photolithographic processes. The conductive trace pattern is photodefined in order to determine the length and width dimensions of the heater resistor areas formed within the tantalum aluminum resistive layer, and this conductive trace pattern further provides electrical lead in connectors to each of the photodefined heater resistor areas in the tantalum aluminum resistive layer.
To complete the composite TIJ printhead structure, a surface dielectric material such as silicon dioxide, SiO.sub.2, silicon nitride, Si.sub.3 N.sub.4, or silicon carbide, SiC, or a composite of the above insulating materials including silicon oxynitride, SiO.sub.x N.sub.y, is then frequently deposited on the exposed surfaces of the aluminum trace material and over the exposed surfaces of the heater resistor areas in order to provide a protective coating over these latter areas. Then, a polymer barrier layer material such as Vacrel is applied and photolithographically patterned on top of this latter surface dielectric material to define the dimensions of the ink drop ejection chambers which are positioned to surround and be coaxially aligned with respect to the previously formed heater resistors. Finally, an orifice plate such as nickel is secured to the top of the polymer barrier layer and has orifice openings therein which are also coaxially aligned with respect to the centers of the ink drop ejection chambers and the centers of the previously formed heater resistors.
During the above printhead manufacturing process, it is possible to separate the individual silicon or glass substrates one from another either before or after the above described orifice plate formation step. This is typically done by dicing through the silicon or glass substrate upon which the above individual printhead devices are constructed. This operation is quite dirty, and the substrates must be protected from contamination and damage during the dicing process. The individual printheads must then be subjected to a cleaning cycle before further assembly operations can take place, and these dicing and cleaning operations add a substantial cost to the printhead manufacturing process. In addition, the quality and cost of the glass or silicon substrates are largely controlled by outside vendors, and this in turn may adversely affect the reliability of and quality control over the printhead batch manufacturing process.
Another prior art process for forming thermal ink jet printheads is described in U.S. Pat. No. 4,616,408 issued to William J. Lloyd and entitled "Inversely Processed Resistance Heater". The Lloyd process describes a resistance heater which contains a relatively thick layer of electroplated metal such as nickel or copper deposited on the order of 10 to 1000 microns in thickness and used to serve as both a heat sink and support layer for the ultimately formed thin film printhead structure. This metal layer must then be bonded to another support bearing substrate, and this process is somewhat complicated in its nature and overall number of process steps used therein.
In addition to the above required dicing and cleaning processes used in the manufacture of the prior art thermal ink jet printheads, the above glass or silicon substrates therefor had to be additionally processed in order to form ink feed holes therein for providing a path of ink flow from a source of ink supply within a pen body housing and into the above described ink drop ejection chambers located around each of the heater resistors. These ink feed holes have been formed using sandblasting and laser drilling processes which are difficult to control and somewhat expensive to carry out. In addition, sandblasting is dirty, imprecise, and can create rough areas on the underlying substrate which tend to absorb ink at undesirable locations. Also, as previously indicated the cutting or dicing processes used to separate multiple printheads fabricated on a common wafer are dirty and they add further costs to the above required laser drilling or sandblasting processes which are used to define the ink feed holes in the substrates.
Once completed, the above described TIJ printheads which utilized either glass or silicon substrates in combination with metal orifice plates exhibited a rather poor thermal match characteristic inasmuch as the thermal coefficient of expansion of the glass or silicon substrate is much smaller than the thermal coefficient of expansion of the metal orifice plate. Such thermal expansion mismatch between substrate and orifice plate can cause bowing in the completed printhead structure and even possibly device failure and mechanical separation therein between the substrate and orifice plate. Moreover, the above problem of mismatch in thermal expansion coefficients between substrate and orifice plate gets worse as the printheads get larger and longer, such as for example in the construction of pagewidth printheads. Such pagewidth printheads are becoming more desirable as a necessary means for making high throughput ink jet printers of the future.